Those problem is those default values are set by configure. For example, in mk/config.mk.in:

SplitObjs=$(SupportsSplitObjs)

The idea of the perf buildflavour is that it matches the default build. This is almost true, except for -H64m vs -H32m and STRIP_CMD=: vs STRIP_CMD=strip (that last one has performance effect actually). I intend to reconcile these differences later.